This invention relates to the detection of flames, in particular in the vicinity of a conventional wet bench of the type used in the production of VLSI integrated circuit chips. These wet benches are located in so-called "clean rooms", areas which are maintained in an essentially contaminent free state.
Conventional wet benches (see FIG. 1) are constructed of polypropylene and include a table-top surface and a series of tubs (in the form of deep depressions in the surface), each tub containing one of several hydrocarbon solvents. Heating elements are located below the tubs and are used to heat the hydrocarbon solvents contained therein. A plenum, spanning the entire length of the bench, is located below the heating elements and serves as a collection area for expended solvents. The plenum also serves as a condensation tank for solvent fumes drawn into the plenum by a constant flow of air from above the wet bench. The volatile nature of the hydrocarbon solvents, in combination with the necessity of heating some of the solvents, results in an above average risk of fire.
Coupled with the increased risk of fire is the extraordinary monetary loss, in terms of clean-up costs and lost production time, which results from clean room fires; these costs are typically in the millions, and possibly even billions, of dollars. The extraordinary size of the loss results from the necessity of shutting down at least the clean room (if not the entire section of the plant in which the clean room is located) and removing the contaminants in the event of fire. It is therefore extremely desirable to detect and suppress any fires which occur in the vicinity of a wet bench, including those which occur in the plenum.
The detection of a flame generated by burning hydrocarbon solvents presents severe technical difficulties. One difficulty is the highly caustic nature of the solvents contained in the tubs of a wet bench. The presence of these solvents effectively precludes the use of a conventional glass window to cover and protect the sensor; a substance must be used which is inert in the presence of these solvents.
Another difficulty is the prevention of false alarms. A detector must be able to distinguish between flames generated by hydrocarbon solvents, to which a response is desired, and other sources of heat and flame, including bunsen burners and electric welding torchs, which a detector should ignore.
As a fire begins to burn, clouds of fumes (from water based chemicals used in chip production) may obscure a detector, preventing it from detecting the presence of the hidden flames. A detector must thus be able to detect a form of radiation which can be "seen" through the clouds (i.e. a form of radiation not readily absorbed by water based chemicals).
In the plenum, additional difficulties arise from the increased concentration of fumes and the great sensitivity which a detector operating in the plenum must possess in order to detect flames occuring the length of the plenum (i.e. up to eight feet away).
Polypropylene is used in construction of the bench because it is inert in the presence of the solvents typically encountered in chip production. Unfortunately, polypropylene melts and burns; when a fire occurs in one of the tubs, one of the first detectable signs may be burning polypropylene droplets dripping into the plenum. It is thus desirable for a detector operating in the plenum to be capable of detecting these burning droplets. It has been observed that when polypropylene burns, droplets occur at a rate of approximately two droplets per second.
Finally, even where the fire is detected and a chemical suppressant released, the possibility exists that the fire may not wholly be extinguished, or even if extinguished, that it may reignite. It is therefore desirable to employ a "multishot" system, capable of detecting and responding to a secondary fire. This requires that the detector be capable of determining when the primary fire is apparently extinguished and, in response, suspending the release of the chemical suppressant, and that the detector be capable of functioning in the presence of the chemical suppressant so as to detect the secondary fire.